Conventionally, in-line self-convergence color picture tubes distort a horizontal deflection magnetic field as a pincushion shape and a vertical deflection magnetic field as a barrel shape. According to the conventional picture tube apparatus, astigmatism of the electron beam caused by the deflection magnetic field is generated and defocus of the electron beam is generated as the distance to the screen becomes long. Consequently, the beam spot can be focused in an optimum condition in the horizontal direction, however, the beam spot is over-focused in the vertical direction and the resolution in the vertical direction is deteriorated.
In order to solve the above-mentioned problem, one structure is disclosed in Japanese Laid Open Patent No. (Ibkkai-Sho) 61-99249. FIG. 10 is a perspective view showing a part of an electron gun of the color picture tube apparatus of the prior art. The electron gun shown in FIG. 10 comprises a cathode 5, a control lattice electrode 6, an accelerating electrode 7, a first focusing electrode 8, a second focusing electrode 9 and a final accelerating electrode 10.
Circular beam through holes are provided in the end surface of the control lattice electrode 6, in that of the accelerating electrode 7, and that of the first focusing electrode 8 facing the accelerating electrode 7. Further, circular beam through holes are provided in the end surface of the second focusing electrode 9 facing the final accelerating electrode 10 and that of the final accelerating electrode 10 facing the second focusing electrode 9.
A nonaxisymmetric electrostatic lens generating means is provided between the first focusing electrode 8 and the second focusing electrode 9. More concretely, electron beam through holes that are vertically oblong are provided at the end surface of the first focusing electrode 8 facing the second focusing electrode 9 and electron beam through holes that are horizontally oblong are provided at the end surface the second focusing electrode 9 facing the first focusing electrode 8.
A compound voltage, in which a dynamic voltage Vd is synchronized with the deflection of electron beam and superimposed on the focusing voltage V.sub.g3, is applied to the second focusing electrode 9.
FIG. 11 shows an example of the lens model of the conventional color picture tube apparatus. The upper side of FIG. 11 shows the horizontal direction and the lower side of FIG. 11 shows the vertical direction. An electron beam trajectory 18 shows the electron beam trajectory at the center part of the screen and the periphery of the screen when the electron beam is deflected.
When an electron beam is deflected, a quadrupole lens 16 is generated by the nonaxisymmetric electrostatic lens generating means and the astigmatism of electron beam caused by the deflection magnetic field is compensated by the quadrupole lens 16. At the same time, a potential of the second focusing electrode 9 is increased and the difference between the potential of the second focusing electrode 9 and the accelerating potential Va of the final accelerating electrode 10 is decreased. As a result, the focusing function of the main lens 17 provided between the second focusing electrode 9 and the final accelerating electrode 10 is weakened and at the same time, the defocus of the electron beam can be compensated.
However, the above-mentioned color picture tube apparatus has the following problems.
(1) As shown in FIG. 12, the distance between the electron gun 3 and the periphery of the phosphor screen 2 is longer than that between the electron gun 3 and the center of the phosphor screen. As a result, .THETA.p, the angle of incidence of electron beam at the periphery of the phosphor screen becomes smaller than .THETA.c, that at the center of the phosphor screen. In general, the magnification of the lens is in inverse proportion to the angle of incidence at the screen, and therefore the diameter of the spot at the periphery of the screen becomes longer than that at the center of the screen. Consequently, when there is the difference of the diameter of the spot between the center and the periphery of the screen, the uniformity of the focus of electron beam between the center and the periphery of the screen is deteriorated.
(2) When the size of the color picture tube apparatus is enlarged, the dynamic voltage is increased. Consequently, if the size of the color picture tube apparatus is intended to be enlarged, the load of the circuit is increased, and as a result, the cost is increased.
(3) Two pins for applying the voltage are required, consequently the load of the circuit is increased, and as a result, the cost is increased.